Ameloblastoma is a benign odontogenic neoplasm of the mandible and maxilla that rarely exhibits malignant behavior. We report the case of an aggressive malignant ameloblastoma of the mandible that presented with an unusual multiphasic, histologic pattern. Initial fine needle aspiration and radiographic findings showed features consistent with a benign, fibro-osseous lesion. However, aggressive growth and the association of enlarged submandibular lymph nodes suggested a more malignant potential. Treatment consisted of an angle-to-angle composite mandibular resection, right modified neck dissection, left functional supraomohyoid neck dissection, and anterior chin skin resection with iliac crest osteocutaneous free flap reconstruction. Microscopic evaluation showed primarily malignant ameloblastoma without cellular atypia and extensive fields of fibro-osseous tissue with smaller fields of clear cell odontogenic tumor. This multiphasic, histologic arrangement may explain the perplexing preoperative microscopic diagnosis, suggesting a benign fibro-osseous lesion. Of the lymph nodes analyzed, one from the right submandibular triangle exhibited metastatic, benign-appearing ameloblastoma without fibro-osseous or clear cell features. The absence of cellular features of malignancy in the tumor mass and lymph node metastasis suggest that the lesion should be classified as malignant ameloblastoma rather than ameloblastic carcinoma or odontogenic carcinoma. A malignant ameloblastoma with all 3 of the aforementioned microscopic features has not been previously reported. We review the classification of epithelial odontogenic malignancies. Lesions showing multiphasic patterns can create diagnostic dilemmas and may require extensive surgical sampling and/or removal to establish an accurate diagnosis.